Microchips are made from extremely pure circular wafers of silicon. This extreme purity of the wafers requires extremely clean processing environments since even extremely small impurities or particle deposits can ruin a wafer. This is becoming more critical as the line size of wafer features become smaller, and even smaller impurities are harmful. Semiconductor manufacturing involves a number of processes, each requiring different tools and equipment, mostly robotic, all within a sealed clean environment. To further protect the wafers from contamination they are transported from station to station using sealed carriers.
Many tools in a semi-conductor manufacturing line require the wafers to be placed in the tool in an exact orientation. Since wafers' orientations are randomly positioned in the carriers, a device must be used to find the orientation of the wafers relative to robots that move the wafers in and out of the tools. This tool is known as a wafer prealigner in the semi-conductor industry. After the robot picks up a wafer from its carrier, it is placed on the prealigner to find the wafer center and orientation. A conventional prior art prealigner utilizes a turntable and a linear CCD array. The linear array is oriented along the radial axis of the turntable, typically centered on the edge of the wafer.
Each piece of equipment in a semiconductor manufacturing line is costly and many operations require a prealigner. Since there are several different diameters of wafer and prealigners are diameter specific, several prealigners may be required for each process. Furthermore, since setting up the prealigner takes time and human intervention, the prealigner can be a bottle neck in a semi-conductor processing line. In prior art prealigners, a wafer is placed on a turntable by a robot, and secured with a vacuum. Then the wafer is spun about the center axis of the turntable, and if the wafer center coincides with the center of the turntable no edge movement will be observed by a linear CCD array. If the wafer center is offset from the center of the turntable, a sinusoidal movement of the wafer edge relative to the linear CCD array can be observed. By processing the sinusoidal movements of the wafer edge relative to the wafer rotation angles, the position of the wafer center can be calculated. After the wafer center is determined, the wafer center is moved to the center of the turntable by using three vertical pins underneath the wafer. The pins rise up and lift the wafer above the turntable and then move laterally to align the wafer and turntable centers. Appropriate combinations of rotation by the turntable and horizontal translation by the pins can move the wafer until it is centered on the turntable. After the centers are aligned, the wafer is spun again, and no edge movement should be observed by the CCD array. Another task that the prealigner performs is to orient the wafer to the proper angular position. Silicon wafers typically have a flat spot or a notch on them. As the wafer is rotated on the turntable, the flat spot or notch will be observed by the CCD array as an abrupt change in the edge position of the wafer. In this way the prealigner is able to measure the angular orientation of the wafer while processing the edge movement data. Once centered, appropriate rotation of the chuck can adjust the wafer to the desired angular orientation.
There are several disadvantages associated with the prior art. The first disadvantage is that the position of the linear CCD array depends on the wafer diameter. The wafer diameter must be given and a prealigner with the proper CCD array can then be installed and used to identify the wafer center and orientation. This makes the conventional wafer prealigner a size dependent device requiring the cost of multiple prealigners or CDD arrays to change between wafer diameters. It would be advantageous to provide a single wafer prealigner that is able to process a variety of wafer diameters using a simpler and less expensive edge sensor than a CCD array.
A second disadvantage of the current art prealigner is that the wafer must be moved by mechanical pins to align the centers. This causes surface contact and particle generation. It would be advantageous to provide a prealigner that centered the wafer without contacting the back of the wafer.
A third disadvantage of the current art prealigner is the large surface contact between the backside center of the wafer and the turntable. As the wafer size grows larger, and the line size of features on the wafer shrinks to sub-micron range, particles generated from physical contact become highly undesirable. It would be advantageous to provide a prealigner which has no need for a vacuum turntable, and which rotates and secures the wafer without physical contact with the center of the wafer.
The very edges of a wafer are often not used due to the inefficiency of laying out square microchips on a round wafer and physical contact here would be more desirable than in the center of the wafer. An alternative design for a prealigner is described in U.S. Pat. No. #4,887,904, the specification of which is incorporated herein by reference, which uses an air-bearing table to support the wafer and only contacts the wafer at the periphery, thus avoiding physical contact with the center of the wafer. The wafer is rotated by one of three rollers which hold the wafer by pressing radially inward on the wafer. This design is not desirable because inward pressure can cause warping or breakage of the wafer. Silicon wafers generally have a ±0.1 mm circularity tolerance which could cause large variations in inward pressure as the wafer is rotated in this design, in addition to the disturbance caused by the notch passing over the rollers, especially the driving roller. The design proposed in the above patent is also limited to a single wafer diameter. It would be advantageous to provide a prealigner which only contacts the wafer at the periphery and does not cause any inward radial pressure.